In prior art solar cells, an electrode is produced by directly plating a low resistance metal on a surface of the solar cells using vacuum evaporation or sputtering or by screen printing a conductive resin on the surface of the solar cells and baking it.
FIG. 10 shows a prior art electrode production method using vacuum evaporation. In this method, a metal mask 20 having an aperture is disposed on a surface 4 of a solar cell 1. A low resistance metal 21 is heated and evaporated by irradiation with an electron beam 23 from an electron gun 22, whereby an electrode comprising a thin metal film is deposited on the surface 4 of the solar cell 1 in a high vacuum.
FIG. 11 shows another prior art electrode production method using screen printing. In this method, a conductive resin 25 is applied to the surface 4 of solar cell 1 through apertures of a screen 26 using printing means 24. The conductive resin 25 is baked, thereby producing an electrode. In this method, the electrode is thicker than in the vacuum evaporation or sputtering methods.
FIG. 12(a) is a plan view of a solar cell having a conventional electrode structure. FIG. 12(b) is a cross-section taken along line XIIb--XIIb of FIG. 12(a). In these figures, an electrode pattern 27 comprising a metal film is formed on the surface 4 of the solar cell 1 by the method described above. A conductive material 28 for conducting the photocurrent from the solar cell 1 is adhered to a predetermined portion of the metal film 27 by solder 29.
The photocurrent flows into the metal film electrode 27, to the conductive material 28, and out of the solar cell. The conductive material 28 is attached to a predetermined portion of the electrode 27 with solder or a conductive resin.
Power losses arise from the series resistances of the surface and the electrode. In order to minimize the power loss, the electrode pattern is designed using various parameters, such as sheet resistance, contact resistance between the surface and the electrode, the width, thickness, and number of electrodes, the resistivity of electrode material, and current density. The desired electrode pattern 27 is produced using the metal mask 20 or the screen mask 26.
Since the prior art electrode is produced by metal evaporation or printing of a conductive resin, the thickness of the electrode is restricted to a range from about several microns to about several tens of microns. Because of the limited thickness of the electrode, in order to minimize the power loss, the electrode needs to be wide or a large number of electrodes needs to be used. For example, when the electrode is produced by evaporation of silver (resistivity .rho.=1.6.times.10.sup.-6 .OMEGA..multidot.cm) to a thickness of 5 microns, in order to obtain a resistance approximately equal to that of a copper wire 50 microns in diameter, the electrode width has to be about 370 microns. When the electrode is produced by screen printing of a conductive resin including silver (resistivity .rho.=5.times.10.sup.-5 .OMEGA..multidot.cm ) to a thickness of 5 microns, the electrode width has to be about 1160 microns. As a result, the area of the electrode on the surface of solar cell is relatively large, reducing the effective light incidence area of the solar cell and reducing the current generating capacity of the solar cell. Moreover, the power loss at these electrodes is undesirably high.